For persons with impaired hearing, the hearing aid devices which are most commonly used today are those based on the principle that the sound is amplified and fed into the auditory meatus and stimulates the eardrum from the outside. In order to prevent acoustic feedback problems in these devices, the auditory meatus is almost completely plugged by a hearing plug or by the hearing aid device itself. This causes the user a feeling of pressure, discomfort, and sometimes even eczema. In some cases it even causes the user problems like running ears due to chronic ear inflammations or infections in the auditory canal.
However, there are other types of hearing aids on the market, i e hearing aids based on another type of sound transmitting principle, specifically bone conducting hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing and it has also been found that it can be a help for persons with stuttering problems. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461 and it is also referred to the BAHA® bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.
Other types of bone conducting hearing aids are described in U.S. Pat. No. 4,904,233 and WO 01/93635.
A common feature for the hearing aid devices which have been described so far is that some type of vibratory generating means, vibrators, are required. Different types of vibrators are well known in the art. There are a number of known vibrator principles today. In traditional as well as in bone anchored hearing aid devices it is normally used a vibrator principle which was described already by Bell in 1876. There is a detailed description of this principle applied on a bone anchored, bone conducting hearing aid device in “On Direct Bone Conduction Hearing Devices”, Technical Report No. 195, Department of Applied Electronics, Chalmers University of Technology, 1990. Other vibrators of this type are described in WO 01/93633, WO 01/93634, U.S. Pat. No. 6,751,334 and PCT/SE03/00751.
A typical vibrator of this type comprises a magnetic device, a vibrator plate and a so-called inner spring member in order to provide an air-gap between the magnetic device and the vibrator plate. In order to reduce the risk for acoustic feed back problems in the hearing aid device it is necessary to damp the resonance frequency of the vibrator. In this context it is referred to Swedish Patent No. 85.02426-3 in which it is described a vibrator which comprises a vibrator plate and a coil which is wound around a bobbin base with a core and two side walls. It also comprises means for damping the resonance frequency of the vibrator such that the spring member is provided with a layer of a damping material or a built-in damping material. The entire vibrator arrangement is housed in a casing and a so-called outer spring in the form of a thin metal plate is arranged between the vibrator arrangement and the casing in order to isolate the movements of the magnetic device relative to the casing. The outer spring is suspended into the vibrator casing via elastic elements, silicon pads or the like.
The vibrator plate is mechanically connected via a vibratory transmitting element to a coupling device, such as a snap-in coupling, a bayonet coupling, a magnetic coupling or the like, for connecting the outer hearing aid part to the bone anchored part of the hearing aid device.
To prevent dust and dirt to come into the hearing aid housing there is a sealing between the casing of the hearing aid and the vibratory transmitting element, for instance an O-ring.
A disadvantage with this type of vibrator arrangement is the fact that it comprises so many small parts which makes it difficult to assemble. The separate suspension of the outer spring and the sealing of the casing comprises small elastic elements which must be robust enough to withstand a long-time use of the hearing aid but also weak and soft enough to serve as a vibratory isolating and dust sealing element.
Another disadvantage with the known arrangement is the fact that the vibratory isolation is not always optimal due to the fact that the outer spring in the form of a small, thin metal plate which is used today is weak in one direction, perpendicular to the plane of the spring plate, but stiff against movements in other directions parallel to the plane of the spring plate and also stiff against rotary movements. Vibratory movements in these directions are absorbed by the silicon pads only.